Semiconductor chip having multiple functional blocks integrated in a single chip and method for fabricating the same

ABSTRACT

A semiconductor chip comprises a base substrate, a bulk device region having a bulk growth layer on a part of the base substrate, an SOI device region having a buried insulator on the base substrate and a silicon layer on the buried insulator, and a boundary layer located at the boundary between the bulk device region and the SOI device region. The bulk device region has a first device-fabrication surface in which a bulk device is positioned on the bulk growth layer. The SOI device region has a second device-fabrication surface in which an SOI device is positioned on the silicon layer. The first and second device-fabrication surfaces are positioned at a substantially uniform level.

[0001] This patent application is based upon and claims the benefit ofthe earlier filing date of Japanese Patent Application No. 2001-298533filed Sep. 27, 2001, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a semiconductor chip having afunctional block positioned in an SOI (silicon-on-insulator) region andanother functional block positioned in a bulk region in a single chipand a method for fabricating such a semiconductor chip.

[0004] 2. Description of Related Art

[0005] DRAM chips having a 1T1C (1-transistor-1-capacitor) memory cellstructure have been widely used as an inexpensive and large-capacitymemory suitable for high-density integrated circuits. In recent years,demand has been increasing for a system LSI, in which a DRAM and a logiccore are integrated in a single chip in order to improve systemperformance.

[0006] On the other hand, SOI devices, such as a SOIMOSFET, using an SOIsubstrate in place of a conventional silicon bulk substrate has beenattracting a great deal of attention. In SOI devices, transistors areformed in the silicon layer positioned on the buried oxide (referred toas “SOI layer”) in an SOI substrate. Such SOI devices have already beenmass-produced for use in high-performance logic circuits. Along withthis trend, in order to further bring out the advantages of ahigh-performance logic circuit consisting of SOI devices (hereinafterreferred to as an “SOI logic”), development of a system LSI or asystem-on-chip which carries a memory (e.g., a DRAM) together with anSOI logic on a single chip has become an urgent necessity.

[0007] However, it is difficult to form a DRAM in an SOI substrate,employing the same structure with the high-performance logic devices(e.g., SOIMOSFETs), for several reasons.

[0008] First, leakage current or fluctuation of the threshold voltagewill occur during operation because electric potential of the substrate(i.e., the body region) of the SOIMOSFET is floating. If such anSOIMOSFET is used as a path-transistor, leakage current (e.g., aparasitic MOSFET current or a parasitic bipolar-current) occursdepending on the operational conditions of the source/drain voltage,even if the gate voltage is in the OFF condition. For this reason, froma viewpoint of retention, the SOIMOSFET structure is unsuitable for DRAMcell transistors having a strict leakage-current spec.

[0009] Second, the threshold voltage varies in accordance with changesin the operational conditions, including operation hysteresis, due tothe floating body effect. Accordingly, if the sense amplifier of theDRAM is comprised of SOIMOSFETs, variation in the threshold voltagebetween the pair transistors is amplified, and the sense margindeteriorates.

[0010] To solve the problem of the floating body effect, a technique forfixing the body potential by providing a contact to the additionaldevice region extracted from the body of the conventional MOSFET patternwas proposed. However, this method increases the occupied area of boththe memory cell and the sense amplifier greatly, and spoils the highintegration feature, which is the main characteristic of a DRAM.

[0011] Then it is proposed to form a bulk substrate region as a portionof an SOI substrate, and to form circuits, such as DRAMs, which areincompatible with the floating body effect, in the bulk substrateregion. In fact, various methods for fabricating a substrate having botha bulk structure and an SOI structure (referred to as an “SOI/bulksubstrate”) have been proposed.

[0012] A first approach is a SIMOX (separation by implanted oxygen)technique using a mask pattern (Japanese Patent Application Laid-open(Kokai) No 10-303385, and Robert Hannon, et al. 2000 Symposium on VLSITechnology of Technical Papers, p66-67). With this method, oxygen isimplanted in predetermined positions in the silicon bulk substrate toproduce an SOI structure that coexists with the silicon bulk region.

[0013] A second approach is a wafer bonding technique for bonding asilicon substrate onto another silicon substrate with a patternedinsulator (Japanese Patent Application Laid-open (Kokai) No. 8-316431).

[0014] A third approach is to etch the SOI layer and the buried oxide ata predetermined position of the SOI substrate to partially expose thebase substrate, thereby producing a bulk region in the SOI substrate(Japanese Patent Application Laid-open (Kokai) Nos. 7-106434, 11-238860,and 2000-91534).

[0015] A fourth approach is to form all epitaxially grown silicon layeron the base substrate in order to eliminate the level difference betweenthe SOI substrate region and the bulk region resulting from the partialetching in the third approach (Japanese Patent Application Laid-open(Kokai) No. 2000-243944). In this method the epitaxial layer is grownuntil it exceeds the mask layer placed over the SOI substrate region,and then it is planarized using the mask layer as a stopper.

[0016] There are problems with these approaches to forming an SOI/bulksubstrate.

[0017] The first approach deteriorates the crystalline characteristic ofthe SOI layer due to the implantation of oxygen ions. In addition,volume expansion that occurs when the buried oxide is formed by reactionbetween silicon and the implanted oxygen in a thermal process causesstress, and crystal defect is produced at the boundary between the SOIsubstrate region and the bulk region.

[0018] The second approach produces an undesirable interface state and acrystal-defect layer, which deteriorate both the crystal characteristicand the electrical characteristic at the bonding surface between the twosubstrates. Such an interface state and crystal defect are due tocontamination and shifting of crystal orientation.

[0019] The third approach causes a level difference between the SOIsubstrate region and the bulk region by an amount corresponding to thethickness of the SOI layer and the buried oxide. This level differencemakes it difficult to guarantee the focusing margin in thephotolithography process, and to control the height of the buriedinsulator in the trench when forming isolations.

[0020] In the fourth approach, the crystal line characteristic of theepitaxial growth layer may deteriorate near the interface between thebulk region and the SOI substrate region. This problem is caused by thefact that crystal grows from both the top face of the base substrate andthe sidewall of the SOI layer during the formation of the bulk growthlayer. The crystal characteristic of the epitaxial layer having grownfrom the etched side face of the SOI substrate is inherently bad. Inaddition, the crystal orientations of the epitaxial layers having grownfrom the top surface of the base substrate and from the sidewalls of theSOI layer are mismatched with each other at the interface between themfurther deteriorating the crystal characteristic.

[0021] Then, it is conceived to cover the exposed sidewall of the SOIlayer with a protection film, such as silicon nitride film, beforeforming the epitaxial growth layer in order to solve the above-describedproblem.

[0022] However, if a sidewall protection film (e.g., Si₃N₄) exists atthe boundary between the epitaxially grown bulk region and the SOIsubstrate region, a relatively large stress is produced in both theepitaxial growth layer and the SOI layer over several micrometers nearthe boundary, depending on the process conditions. Such stress may causechange in the mobility of the carriers and crystal defect. If atransistor is positioned in an area having crystal defect, the devicecharacteristic becomes inferior.

[0023] Furthermore, because the epitaxial growth layer is polished usingthe mask layer as a stopper, the final level of the epitaxial growthlayer close to the boundary in the bulk region becomes higher than theSOI layer of the SOI substrate region equivalent to the thickness of themask layer. To avoid the surface unevenness, a troublesomeafter-treatment, for example, re-polishing the epitaxial growth layerafter thinning the mask layer, must be carried out. If the epitaxialgrowth layer is set broad in order to form a DRAM macro in it, dishing,which is a phenomenon where a center portion of the layer sinks, occurs.The unevenness of the top surface remains as a step or a leveldifference in the subsequent processes, and adversely affects themanufacturing process.

[0024] Therefore, a novel and improved approach to solving theseproblems in the conventional methods is desired.

SUMMARY OF THE INVENTION

[0025] In one aspect of the invention, a semiconductor chip comprises abase substrate, a bulk device region located on a part of the basesubstrate and having a bulk growth layer, an SOI device region locatedon the other part of the base substrate and having a buried insulatorand a silicon layer located on the buried insulator, and a boundarylayer located between the bulk device region and the SOI device region.The bulk device region has a first device-fabrication surface in which abulk device is fabricated, and the SOI device region has a seconddevice-fabrication surface in which an SOI device is fabricated. Thefirst and second device-fabrication surfaces are positioned atsubstantially the same level.

[0026] In another aspect of the invention, a method for fabricating asemiconductor chip comprises (a) preparing an SOI substrate consistingof a base substrate, a buried insulator on the base substrate, and asilicon layer an the buried insulator, (b) removing a portion of thesilicon layer and the buried insulator at a predetermined region of theSOI substrate, (c) forming a sidewall protection film covering the sideface of the silicon layer exposed by the removal (d) exposing the basesubstrate at said predetermined region, and forming a bulk growth layeron the base substrate so as to be in alignment with the top face of thesilicon layer, (e) forming isolations in the bulk growth layer and theSOI substrate, the isolations having the same depth, and (f) formingdevices in the bulk growth layer and the SOI substrate.

[0027] In still another aspect of the invention, a method forfabricating a semiconductor chip comprises (a) preparing an SOIsubstrate consisting of a base substrate, a buried insulator on the basesubstrate, and a silicon layer on the buried insulator, (b) removing aportion of the silicon layer at a first position on the SOI substrateand forming a first isolation in the removed portion, (c) exposing thebase substrate at a second position, while keeping a side face of thesilicon layer covered with the first isolation, (d) forming a bulkgrowth layer from the exposed base substrate so as to be in alignmentwith the top face of the silicon layer, (e) forming a second isolationin the bulk growth layer, the second isolating being deeper than thefirst isolation, and (f) forming devices in the bulk growth layer andthe SOI substrate

[0028] In yet another aspect of the invention, a method for fabricatinga semiconductor chip comprises (a) preparing an SOI substrate consistingof a base substrate, a buried insulator on the base substrate, and asilicon layer on the buried insulator, (b) removing a portion of thesilicon layer and the buried insulator at a predetermined region on theSOI substrate to expose the base substrate, (c) forming a first part ofa trench capacitor having a first width in the exposed base substrate,(d) forming a bulk growth layer from the base substrate so as to be inalignment with the top face of the silicon layer, and (e) forming asecond part of the trench capacitor having a second width in the bulkgrowth layer, the second width being smaller than the first width.

[0029] In yet another aspect of the invention, a method for fabricatinga semiconductor chip comprises (a) preparing an SOI substrate consistingof a base substrate, a buried insulator on the base substrate, and asilicon layer on the buried insulator, (b) removing a portion of thesilicon layer and the buried insulator at a predetermined region on theSOI substrate to expose the base substrate, (c) forming a bulk growthlayer from the exposed base substrate so as to be in alignment with thetop face of the silicon layer, (d) forming a dummy pattern layer in thebulk growth layer near the boundary between the bulk growth layer andthe SOI substrate, the dummy pattern layer being deeper than the buriedinsulator of the SOI substrate, and (e) forming devices in the bulkgrowth layer and the SOI substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 illustrates examples of the plan layout of a semiconductorchip, to which the present invention is applied.

[0031]FIG. 2 is a schematic cross-sectional view of the semiconductorchip according to the first embodiment of the invention.

[0032]FIGS. 3A through 3G illustrate a fabrication process of thesemiconductor chip shown in FIG. 2.

[0033]FIG. 4 is a schematic cross-sectional view of the semiconductorchip according to the second embodiment of the invention.

[0034]FIGS. 5A through 5C illustrate a fabrication process of thesemiconductor chip shown in FIG. 4.

[0035]FIG. 6 is a schematic cross-sectional view of the semiconductorchip according to the third embodiment of the invention.

[0036]FIGS. 7A through 7F illustrate a fabrication process of thesemiconductor chip shown in FIG. 6.

[0037]FIGS. 8A through 8C illustrate another fabrication process of thesemiconductor chip shown in FIG. 6, where the steps subsequent to FIG.8C are the same as those shown in FIGS. 7D through 7F.

[0038]FIGS. 9A and 9B are schematic cross-sectional views ofsemiconductor chips according to the fourth embodiment of the invention.

[0039]FIG. 10 is a schematic cross-sectional view of the semiconductorchip according to the fifth embodiment of the invention.

[0040]FIGS. 11A through 11G illustrate a fabrication process of thesemiconductor chip shown in FIG. 10.

[0041]FIG. 12 illustrates a modification of the semiconductor chip shownin FIG. 10.

[0042]FIG. 13 is a schematic cross-sectional view of the semiconductorchip according to the sixth embodiment of the invention.

[0043]FIG. 14 illustrates an example of an arrangement of the dummycapacitors used in the semiconductor chip shown in FIG. 13.

[0044]FIG. 15 illustrates modifications of the dummy pattern used in thesemiconductor chip shown in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

[0045]FIG. 1 illustrates examples of the plan layout of a semiconductorchip 10, to which the present invention is applied. The semiconductorchip 10 has a bulk device region 11, in which circuit elements arepositioned in the bulk substrate domain, and an SOI device region 12, inwhich circuit elements are positioned in the SOI substrate domain. Thesemiconductor chip 10 is a so-called system-on-chip having multiplefunctional blocks in a single chip.

[0046] In the SOI device region 12, transistors are formed in thesilicon layer (i.e., the SOI layer) located on the buried insulator. TheSOI device region 12 is suitable for fabricating circuit elements thatrequire high-speed operations with low power consumption because theinsulator exists directly below the active layer, thereby reducing thejunction capacitance. Examples of such a high-speed circuit elementinclude a logic device. On the other hand, the bulk device region 11 issuitable for fabricating those devices that require a bulk structure inorder to avoiding the floating body effect and the associated problems.Examples of such devices include a DRAM cell and a sense amplifier.

[0047]FIG. 1A illustrates an example where a single bulk device region11 is arranged in the semiconductor chip 10, and FIG. 1B illustrates anexample where multiple bulk device regions 11 are arranged in thesemiconductor chip 10. If DRAM cells are provided as the circuitelements of the bulk device region 11, not only a DRAM array, but alsothe peripheral circuits (e.g., a sense amplifier, a power sourcecircuit, a decode circuit, an I/O circuit, and combinations thereof) arearranged within the bulk device region 11 to form a functional block asa whole. The functional block including the DRAM and the associatedcircuits is referred to as a “DRAM macro”. If MOS transistors are formedin the SOI layer in the SOI device region 12, a high-speed logic circuitis constituted. This logic circuit is referred to as “SOI logic”.

[0048] In order to realize the system-on-chip shown in FIG. 1, aso-called SOI/bulk substrate is required. SOI/bulk substrate has an SOIregion and a bulk region on the same base substrate. The conventionalmethods for fabricating an SOI/bulk substrate involve various problems,as have been described above.

[0049] To avoid the influence of stress on the devices, an adequatemargin space must be guaranteed between the epitaxial bulk region andthe SOI substrate region. In such a case, the margin space, in whichdevices cannot be formed, is dead space, and consequently, the chip sizebecomes large. Meanwhile, it is desirable to eliminate the leveldifference between the epitaxial bulk region and the SOI substrateregion, and to form circuit elements at a uniform level in both regions.

[0050] Therefore, improved semiconductor chips and the fabricatingmethods are provided, which can eliminate the stress between the bulkregion and the SOI substrate region without increasing chip size, andwhich allow the circuit elements to be positioned at a uniform level. Inthe following, various embodiments will be described using an example ofmounting a DRAM macro and an SOI logic on a single chip.

[0051] <First Embodiment>

[0052]FIG. 2 illustrates a semiconductor chip 10 according to the firstembodiment of the invention. The semiconductor chip 10 is depicted in across-sectional view taken along the A-A′ line shown in FIG. 1B. Thesemiconductor chip 10 includes a base substrate 21, a bulk device region11 located on a part of the base substrate 21, an SOI device region 12located on the other part of the base substrate 21, and a polysiliconlayer 47 located at the boundary between the bulk device region 11 andthe SOI device region 12. The bulk device region 11 has a bulk growthlayer 26 positioned on the base substrate 21, in which devices arefabricated. The SOI device region 12 has a buried oxide 22 positioned onthe base substrate 21, and a silicon (SOI) layer 23 located on theburied oxide 22, in which devices are fabricated. In the example shownin FIG. 2, the bulk growth layer 26 is an epitaxially grown singlecrystal silicon layer, and the base substrate 21 is, for example, ap-type silicon base.

[0053] The bulk device region 11 includes a DRAM cell 43 with a trenchcapacitor 30, and a MOSFET transistor 44 for necessary peripheralcircuits (not shown). These devices and circuits as a whole constitute aDRAM macro as a functional block.

[0054] The SOI device region 12 includes an SOIMOSFET 45, whichconstitutes SOI logic.

[0055] The top face of the epitaxial growth layer 26, in which the DRAMcell 43 and the peripheral MOSFET 44 are fabricated, substantiallyaligns with the top face of the SOI layer 23, in which the SOIMOSFET 45is fabricated. Accordingly, the devices or the circuit elements locatedin the bulk device region 11 and those in the SOI device region 12 arepositioned at substantially the same level.

[0056] The DRAM cell 43, the peripheral MOSFET 44, and the SOIMOSFET 45have gate electrodes 39 a 41, and 39 b, respectively, which are made of,for example, polysilicon. The boundary layer 47 between the bulk deviceregion 11 and the SOI device region 12 is polysilicon layer in theexample shown in FIG. 2; however, it may be made of other silicon groupmaterials, such as amorphous silicon or silicon germanium (SiGe). Inorder to reduce the number of fabrication steps, it is preferable to usea gate material for the devices 43, 44, and 45 as the boundary layer 47.In this case, the boundary layer 47 can be provided via a gatedielectric film.

[0057] The top face of the polysilicon boundary layer 47 slightlyretreats from the epitaxial growth layer 26 and the SOI layer 23 in theexample of FIG. 2. However, the boundary layer 47 may project from thesurface of the epitaxial growth layer 26 and the SOI layer 23 up to theheight of the gate 39 a, 39 b, and 41 of the devices 43, 45, and 44, asindicated by the ghost line in FIG. 2. The gate electrodes 39 a, 39 b,and 41 may further have a silicide deposited on the polysilicon.

[0058] The semiconductor chip 10 also has first isolations 35 aisolating the devices 43 and 44 in the bulk device region 11, in whichthe DRAM macro is formed, and second isolations 35 b isolating thedevices 45 in the SOI device region 12, in which the SOI logic isformed. The first isolation 35 a in the bulk device region 11 and thesecond isolation 35 b in the SOI device region 12 are almost the samedepth. To reduce the number of fabrication steps, it is desirable thatthe first and second isolations 35 a and 35 b are formed at the sametime.

[0059] In the example show in FIG. 2, the thickness of the buried oxide22 is relatively large, and the second isolation 35 b reaches to halfwaythrough the buried oxide 22. However, if the buried oxide 22 is not asthick, the second isolation 35 b may reach through to the silicon basesubstrate 21 as long as it is substantially as deep as the firstisolation 35 a, penetrating the SOI layer 23. By setting the secondisolation 35 b to be as deep as the first isolation 35 a, the effectivedistance between two adjacent devices 45 located in the SOI layer 23with the second isolation 35 b between them is lengthened. In this case,the effective distance is the length from the MOSFET 45 to the adjacentMOSFET 45 (not shown) along the contour of the second isolation 35 b.This arrangement can realize miniatured isolation with little leakagecurrent from the interface of the damaged buried oxide 22, and preventdeterioration of the endurance of the second isolation 35 b due to thecurrent leakage. If the first and second insulators are of the samedepth and made of the same material, they can be fabricated at once witha sufficient margin under the same condition.

[0060]FIGS. 3A through 3G illustrate a fabrication process of thesemiconductor chip 10 shown in FIG. 2.

[0061] (a) First, as shown in FIG. 3A, an SOI wafer 20 consisting of asilicon base substrate 21, a buried oxide 22, and an SOI layer 23 isprepared, and mask pattern 24 is formed on the SOI layer 23. The maskpattern 24 is formed of silicon dioxide (SiO₂), silicon nitride (SiN,Si₃N₄, etc.), or a composite layer of these materials.

[0062] (b) Then, as shown in FIG. 3B, portions of the SOI layer 23 andthe buried oxide 22 that are not covered by the mask pattern 24 areremoved by anisotropic etching, such as RIE (Reactive ion etching). Theetching is terminated so that a thin buried oxide 22′ is left on thebase substrate 21, for the purpose of preventing mechanical damage orplasma damage to the silicon base substrate 21.

[0063] (c) Then, as shown in FIG. 3C, a sidewall protection film 25 isformed in order to cover the exposed side face of the SOI layer 23. Thesidewall protection film 25 is, for example, silicon nitride (Si₃N₄) orsilicon dioxide (SiO₂). After such a material is deposited over theentire surface, only the sidewall protection film 25 is left by RIE. Thethickness of the sidewall protection film 25, as well as that of theburied oxide 22 to be removed, are controlled so that the remainingburied oxide 22′ is maintained on the base substrate 21 during theformation of the sidewall protection film 24.

[0064] (d) Then, as shown in FIG. 3D, the remaining buried oxide 22′ isremoved by wet etching using an etchant, such as NH₄F or HF, to exposethe silicon base substrate 21 without damage. Then, after apredetermined pretreatment, a single crystal silicon layer 26 is formedon the exposed silicon base substrate 21 by selective epitaxial growth(e.g., chemical vapor deosition).

[0065] If the sidewall protection film 25 is made of silicon dioxide(SiO₂) in the previous step, the sidewall protection film 25 may also beslightly etched when removing the remaining buried oxide 22′ by wetetching. However, by sufficiently reducing the thickness of theremaining buried oxide 22′ in the step shown in FIG. 3B, silicon basesubstrate 21 is exposed without revealing the side face of the SOI layer23.

[0066] If the sidewall protection film 25 is made of silicon nitride(Si₃N₄) in the previous step, process controllability is improved. Inthe first embodiment, the sidewall protection film 25 is to be removedin a later step. Accordingly, even if the sidewall protection film 25 ismade of Si₃N₄, it will not cause serious stress near the boundary in thefinal product, and priority can be given to process controllability.

[0067] (e) Then, as shown in FIG. 3E, the mask pattern 24 remaining onthe SOI layer 23 is removed, and a mask layer 27 is newly formed overthe entire surface. If the former mask pattern 24 is made of Si₃N₄, itcan be removed using phosphoric acid. If the former mask pattern 24 ismade of SiO₂, it can be removed by hydrogen fluoride (HF). The sidewallprotection film 25 can also be etched depending on the material;however, the etched portion is to be filled with the new mask layer 27,which is also made of SiO₂, Si₃N₄, or a composite layer of thesematerials. The mask layer 27 is patterned, and the DRAM trench capacitor30 is formed using the mask pattern 27 using a technique desired. Forexample, a trench is formed by RIE or other suitable methods, a lowerdiffusion plate electrode 31 is formed, and the trench is filled with,for example, n-type polysilicon via a dielectric film (not shown) toform a storage electrode 29. A collar oxide 32 is formed, and the trenchis further filled with polysilicon. A strap 33 is formed forelectrically connecting the electrode 29 to the n-source/drain 40 a(FIG. 2) of the cell transistor, and the trench is finally filled with,for example, n-type polysilicon.

[0068] (f) Then, as shown in FIG. 3F, isolations 35 a and 35 b areformed in the bulk device region 11 and the SOI device region 12collectively. To be more precise, the mask layer 27 is patterned, andtrenches with the same depth are formed for isolation in both the bulkdevice region 11 and the SOI device region 12. These trenches are filledwith an insulator using the masks 27 as stoppers. In this manner, thefirst isolations 35 a in the bulk device region 11 and the secondisolations 35 b in the SOI device region 12 are formed at the same time.

[0069] If an etching condition that silicon and silicon dioxide areetched at the same etching rate is set, the trenches for the first andsecond isolations in the bulk device region 11 and the SOI device region12 can be dug at the same rate until the trenches reach the depthnecessary to make the isolations between the straps 33 of the DRAMcells, between transistors of the peripheral circuit, and between theSOI devices. The trenches are then filled with the same dielectricmaterial, thereby completing the first and second isolations 35 a and 35b.

[0070] (g) Finally, as shown in FIG. 3G, the buried insulator in thetrench is etched back approximately to the top faces of the SOI layer 23and the epitaxial growth layer 26. The mask layer 27 is also removed.Even in case that the sidewall protection film 25 remains, it is removedduring the removal of the mask layer 27 and in an additional etchingprocess carried out if necessary, and a recess 46 is formed. Then, therecess 46 is filled with a silicon group material to produce theboundary layer 47 shown in FIG. 2. Filling the recess 46 may be carriedout as an independent step, or alternatively, the recess 46 may befilled up when gate electrodes 39, 41 are formed. In the former case,wells and channels are formed, if necessary, using an ion implantationtechnique after the recess 46 is filled. Gate electrodes 39, 41 areformed via the gate dielectrics 48, and source and drains 40 and 42 areformed. In the latter case, the gate electrodes 39, 41 are made of asilicon group material, such as polysilicon or SiGe, and the recess 46is automatically filled with the gate material when forming the gateelectrodes. In either case, the SOI layer 23 is coupled with the singlecrystal silicon epitaxial growth layer 26 via the same silicon groupmaterial between them.

[0071] After the gate electrodes 39, 41 are fabricated, interleveldielectrics and interconnections are formed according to a desiredMOSFET fabrication process. Thus, a semiconductor chip having a DRAMmacro and an SOI logic core in a single chip is fabricated. If asalicide is provided over the gate electrodes and sources and drainswhen forming transistors, it is preferable to protect the boundary witha mask in the structure shown in FIG. 2 in order to prevent deformationof the polysilicon layer 47. If the polysilicon layer 47 projects fromthe device-fabrication surface overlapping the source and drain, therecess is protected by the polysilicon itself, and therefore, salicideprocess can be carried out without an additional protection mask.

[0072] In the example shown in FIG. 2, the buried oxide 22 is relativelythick, and the interface between the base substrate 21 and the epitaxialgrowth layer 26 is located deeply. Accordingly, the pn junction of thestrap 33 and the source and drain of the DRAM cell 43 can be positionedshallower than the interface between the base substrate 21 and theepitaxial growth layer 26 for the purpose of reliably separating the pnjunctions from the interface. This arrangement can prevent junctionleakage and maintain the retention characteristic of the memory cell,even if the interface state deteriorates due to process conditions.

[0073] Since, in the first embodiment, the epitaxial growth layer 26 ofthe bulk device region 11 ad the SOI layer 23 of the SOI device region12 are coupled with each other via a silicon group material, such aspolysilicon or SiGe, stress at the boundary is reduced. Consequently,crystal defect due to stress is prevented.

[0074] Changes in mobility due to stress between two regions are alsoprevented, and those devices located near the boundary can beeffectively protected from deterioration.

[0075] The boundary layer is located at the position where the sidewallprotection film for covering the SOI layer used to exist, and therefore,increase in chip size is prevented. In addition, the device-fabricationsurfaces of the bulk device region and the SOI device region are locatedat the same level, which is advantageous for the subsequent processesfor fabricating trench isolations and gate electrodes using alithography technique.

[0076] The second isolation in the SOI device region 12 is at the samedepth as the first isolation in the bulk device region 11. Consequently,the second isolation in the SOI device region 12 can effectively preventleakage current from the interface of the buried oxide.

[0077] The system-on-chip shown in FIG. 2, which has a DRAM macro andSOI logic on a single chip, is capable of high-speed data transferbetween them at a reduced level of power consumption.

[0078] The bulk device region 11 includes not only the DRAM cells 43,but also the peripheral circuit 44 which form a functional block (i.e.,a DRAM macro) as a whole, and consequently, the circuit design or thedevice design originally developed for a bulk substrate is applicable asit is to an SOI/bulk substrate. Of course, the bulk device region of theSOI/bulk substrate may include other functional macros developed for abulk substrate, such as an analogue circuit macro, ahigh-breakdown-voltage circuit macro, and memory macro other than DRAM.In such a case, the circuit designs for these macros can be applied tothe SOI/bulk substrate to form a system-on-chip semiconductor chip.

[0079] Using the fabrication method of the first embodiment, theisolations in both the bulk device region and the SOI device region,whose device-fabrication surfaces are at substantially the same level,are formed at once at the same etching rate so as to have the samedepth. Accordingly, the process conditions, such as the thickness of thedielectric for filling the trench and the etchback time, aresubstantially the same over the bulk device region and the SOI deviceregion. Consequently, the process for fabricating the isolations issimplified.

[0080] <Second Embodiment>

[0081]FIG. 4 illustrates a semiconductor chip 50 according to the secondembodiment of the invention. The semiconductor chip 50 includes a basesubstrate 51, a bulk device region 11 having an epitaxial growth layer56, and an SOI device region 12 having, a buried oxide 52 positioned onthe base substrate 51 and an SOI layer 53 on the buried oxide 52. Thebulk device region 11 has a first device-fabrication surfaces, in whichdevices 43, 44 are positioned. The SOI device region 12 has a seconddevice-fabrication surface, in which MOSFETs 45 are positioned. Thesemiconductor chip 50 also includes a first isolation 65 a isolating thedevices 43, 44 in the bulk device region 11, a second isolation 65 bisolating the SOIMOSFETs 45 in the SOI device region 12, and a thirdisolation 65 c located at the boundary between the bulk device region 11and the SOI device region 12. In this embodiment, the third isolation 65c is the boundary layer.

[0082] In the second embodiment, the device, 43, 44 and other circuitelements (not shown) constitute a DRAM macro in the bulk device region11, and the SOIMOSFETs 45 constitute an SOI logic in the SOI deviceregion 12, as in the first embodiment.

[0083] The first, second and third isolations 65 a, 65 b, and 65 c areof the same depth and made of the same dielectric material. Theisolations 65 a, 65 b and 65 c are deeper than the buried oxide 52 ofthe SOI device region 12. The first device-fabrication surface of thebulk device region 11, in which the DRAM cell 43 is formed, is insubstantial alignment with the second device-fabrication surface of theSOI device region 12, in which the SOIMOSFET 45 is positioned.Consequently, the devices 43, 44 that constitute the DRAM macro and thedevices (SOIMOSFET) 45 that constitute the SOI logic are positioned atsubstantially the same level.

[0084] The bulk device region 11 has the epitaxial growth layer 56 ofsingle crystal silicon, as a bulk growth layer. The SOI device region 12has the buried oxide 52 and the SOI layer 53 over the silicon basesubstrate 51. The total thickness of the buried oxide 52 and the SOIlayer 53 is set at less than in the structure of the first embodiment.By reducing the thickness of the buried oxide 52, heat generated duringoperation of the SOI device escapes to the base substrate 51effectively. This arrangement is desirable when deterioration of devicecharacteristics due to heat has to be prevented.

[0085] Since the third isolation 65 c is deeper than the buried oxide52, it can prevent crystal defects, such as dislocation, generated atthe boundary from spreading into the epitaxial growth layer 56.

[0086]FIGS. 5A through 5C illustrate a fabrication process of thesemiconductor chip 50 shown in FIG. 4. FIG. 5A follows the step shown inFIG. 3D. Steps 3A through 3D are in common with those in the first andsecond embodiments, except for the thickness of the buried oxide, andthe explanation for them will be omitted.

[0087] In the first embodiment, the problem of stress was solved byremoving the sidewall protection film and by filling the boundary with asilicon group material. However, if the epitaxial growth layer incontact with the sidewall protection film has already been damaged, themeasure in the first embodiment is insufficient.

[0088] Then, in the second embodiment, to remove the crystal damageitself, an SOI substrate with a buried oxide thinner than that used inthe first embodiment is used, or the depth of the isolations includingone located at the boundary are set deeper than the buried oxide. Thesidewall protection film and the area containing deteriorated crystalnear the boundary are removed altogether when forming trenches for theisolations.

[0089] To be more precise, after the epitaxial growth layer 56 is formedin the bulk device region, as shown in FIG. 3D, mask layer 57 is formedover the entire surface covering the epitaxial growth layer 56, the SOIlayer 53, and the sidewall protection film 55. The mask layer 57 ispatterned into a predetermined pattern, and DRAM trench capacitors 30are formed, as shown in FIG. 5A.

[0090] Then, as shown in FIG. 5B, isolations 65 a, 65 b, and 65 c areformed in the bulk device region 11, the SOI device region 12, and atthe boundary between them at the same time. If an etching conditionwhere silicon, polysilicon, silicon nitride, and silicon dioxide areetched at the same rate is selected, trenches with the same depth can beformed at once in the same etching time. All the trenches are deeperthan the buried oxide 52 of the SOI device region 12. Because thesidewall protection film 55 at the boundary is no deeper than the buriedoxide 52, the sidewall protection film 55 and the area containingcrystal defects near the boundary are removed altogether when formingthe isolation trenches. The first isolation 65 a in the bulk deviceregion (or the DRAM macro) 11, the second isolation 65 b in the SOIdevice region (or the SOI logic) 12, and the third isolation 65 c at theboundary are formed at once by filling the trenches with the samedielectric material.

[0091] Then, as shown in FIG. 5C, the insulating layer filled in thetrench is etched back, and the mask layer 57 is removed. Devices 43, 44,45 are formed at predetermined positions, and the semiconductor chip 50illustrated in FIG. 5 is accomplished.

[0092] In the second embodiment, the isolation 65 c located at theboundary is deeper than the buried oxide 52, and the sidewall protectionfilm 55 and the area containing crystal defect near the boundary areremoved altogether when fabricating the isolation. With thisarrangement, stress is reduced in the final product, and crystal defect,such as dislocation, is prevented from spreading from the boundary tothe epitaxial growth layer 56.

[0093] This arrangement has the additional advantage that conventionalisolation technique is applicable directly to the boundary treatment.The advantages of the prevention of increase in chip size and of theeven and uniform device-fabrication surface are the same as those in thefirst embodiment.

[0094] <Third Embodiment>

[0095]FIG. 6 schematically illustrates the semiconductor chip 70according to the third embodiment of the invention. The semiconductorchip 70 comprises a bulk device region 11, in which a DRAM cell 83 and aperipheral MOSFET 84 are positioned, and an SOI device region 12, inwhich a SOIMOSFET 85 is positioned. The bulk device region 11 includesfirst isolations 79, and the SOI device region 12 includes secondisolations 75 a, 75, which are shallower than the first isolations 79.

[0096] Whichever the first or the second isolation that is positionedclosest to the boundary functions as a boundary layer. In the exampleshown in FIG. 6, second isolation 75 a located at the boundary in theSOI devoice region 12 becomes the boundary layer, and it is in contactwith the buried oxide 72 at the bottom. Of course, first isolation 79located at or nearest to the boundary in the SOI device region 11 can bethe boundary layer, depending on the design. In such a case, firstisolation 79 located at the boundary overlap the endmost first isolation75 a and a portion of the buried oxide 72 under the isolation 75 a,although this configuration is not illustrated in the drawing.

[0097] The bulk device region 11 has an epitaxial growth layer 76 as abulk growth layer. DRAM cell 83, and the peripheral MOSFET 84 positionedin the epitaxial growth layer 76 constitute a DRAM macro. The SOI deviceregion 12 has an SOI layer 73 and a buried oxide 72 over the siliconbase substrate 71. MOSFETs 85 formed in the SOI) layer 73 constitute SOIlogic. These devices are positioned at a uniform level over the bulkdevice region 11 and the SOI device region 12.

[0098] The semiconductor chip 70 of the third embodiment has the optimumisolations suitable for the bulk device region 11 and the SOI deviceregion 12 independently, and therefore, the depth of the first isolation79 and the second isolation 75 differ from each other. Either the firstor second isolation, whichever is located at or nearest to the boundary(e.g, second isolation 75 a in the SOI device region 12 in the exampleof FIG. 6), functions as the boundary layer between the bulk deviceregion 11 and the SOI device region 12.

[0099] The optimum arrangement of isolation is desirable when the SOIlogic requires particularly miniaturized pattern. In fact, logic designoften requires miniature isolations. In order to form a trench deeperthan the buried oxide in the SOI substrate in a circumstance where highminiaturization is required, the taper angle of the trench must beprecisely controlled during etching of the side faces of the SOI layerand the buried oxide. In addition, if the post-treatment etches eitherthe SOI layer 73 or the buried oxide 72 too much, the side faces of thetrench become uneven. Without precise angle control in the angle or thetreatment, a void is left inside the trench even after the trench isfilled with a dielectric material. After the surface of the dielectricmaterial is etched, the void can change to a hollow. In this case, whenfabricating gate electrodes, the gate material will enter the hollow,which may cause short-circuit or connection error.

[0100] For this reason, the optimum isolation is provided to the SOIdevice region 12 in the third embodiment. This arrangement has anadvantage of guaranteeing reliable operation, while preventingshort-circuit of the gate electrode or connection error, in addition tothe aforementioned advantages of the reduction of stress, prevention ofincrease in chip side, and uniform level of device-fabrication surfaces.

[0101] In the semiconductor chip 70, whichever the first or the secondisolation located nearest to the boundary functions as the boundaryregion. Accordingly, devices can be arranged at the closest possibleposition to the boundary, and dead space can be greatly reduced.

[0102]FIGS. 7A through 7F illustrate a fabrication process of thesemiconductor chip 70.

[0103] (a) First, as shown in FIG. 7A, shallow isolation 75 is formedinside the SOI device region 12, and another shallow isolation 75 a isformed at the boundary over both the bulk device region 11 and the SOIdevice region 12. To be more precise, a mask material, such as siliconnitride, is deposited over the SOI substrate consisting of silicon basesubstrate 71, buried oxide 72, and SOI layer 73. The mask material ispatterned into a first mask 74. Shallow trenches are formed by RIE atthe exposed positions, which are then filled with a dielectric material,such as SiO₂, to form second isolations 75 and 75 a.

[0104] (b) Then, as shown in FIG. 7B, a second mask material (e.g.,resist) is placed over the entire surface, and is patterned into asecond mask 77 so as to cover the SOI device region 12 and the isolation75 a positioned at the boundary. Using the second mask 77, portions ofthe first mask 74, the SOI layer 73, the boundary isolation 75 a, andthe buried oxide 72 are etched. Preferably, the first mask 74, the SOIlayer 73, the boundary isolation 75 a, and down to halfway through theburied oxide 72 are removed by RIE, and the silicon base substrate 71 isfinally revealed by wet etching.

[0105] It is not necessary to form a sidewall protection film forprotecting the SOI layer 73 because the side face of the SOI layer 73 isalready covered with the second isolation 75 a located at the boundary.Since both the buried oxide 72 and the second isolation 75 a areSiO₂-related composite layers, the etching process is switched to wetprocessing at the final moment for revealing the silicon base substrate71. Wet process allows portions of both the buried oxide 72 directly onthe base substrate 71 and the second isolation 75 a projecting into thebulk device region 11 to be removed without damaging the base substrate71.

[0106] (c) Then, as shown in FIG. 7C, the second mask 77 is removed, anda single crystal silicon bulk layer 76 is formed on the exposed siliconbase substrate 71 by epitaxial growth until it reaches the level of theSOI layer 73, thereby defining a bulk device region 11 in the SOIsubstrate.

[0107] (d) Then, as shown in FIG. 7D, the first mask 74 is removed ifnecessary, and a new mask 78 is formed. Using the mask pattern 78, atrench capacitor 30 is fabricated in the bulk device region 11 asdescribed in the first embodiment.

[0108] (e) Then, as shown in FIG. 7E, first isolation 79, which isdeeper than the second isolation 75, is formed in the bulk device region11.

[0109] (f) Finally, as shown in FIG. 7F, the dielectric filled in thetrench is etched back, and the mask 78 is removed. Transistors 83, 84,and 85 constituting a DRAM and SOI logic are fabricated on the bulkdevice region 11 and the SOI device region 12.

[0110]FIGS. 8A through 8C illustrate an alternative process forfabricating the semiconductor chip 70. In the process shown in FIGS. 7Athrough 7F, the shallow isolations 75 and 75 a were formed only in theSOI device region and at the boundary, and the area that is to be a bulkdevice region was covered with the mask 74 prior to forming the bulkdevice region. In the process shown in FIG. 8, isolation 75 a is formedover the entire area that is to be the bulk device region.

[0111] First, as shown in FIG. 8A, a first mask layer is placed over theentire surface of the SOI layer 73, and is patterned into a first mask74 so as to cover only those areas that are to become active areas inthe SOI device region 12. The SOI layer 73 of the uncovered areas (i.e.,the area that is to be a bulk device region 11 and the areas that are tobe the isolations in the SOI device region 12) is removed, anddielectric layers 75 and 75 a are formed.

[0112] Then, as shown n FIG. 8B, a second mask (e.g., a resist) 77 isprovided over the SOI device region 12 and the portion of the bulkdevice region 11 near the boundary. In other words, the second mask 77covers the entire area of the SOI device region 12 and slightly projectsinto the bulk device region 11. The isolation 75 a and the buried oxide72 are removed continuously by wet etching using the mask 77. Since aportion of the isolation 75 a remains at the boundary, the silicon basesubstrate 71 is revealed with the side face of the SOI layer 73automatically protected by the boundary isolation 75 a, without damagingthe silicon base substrate 71.

[0113] Then, as shown in FIG. 8C, an epitaxial growth layer 76 is formedon the exposed base substrate 71 by selective epitaxial growth.

[0114] The subsequent steps are the same as those in FIGS. 7D through7F. Using the method shown in FIG. 8, the top face of the silicon basesubstrate 71 can be revealed by a single step of wet etching.Accordingly, even if the optimum isolations are fabricated in the bulkdevice region 11 and the SOI device region, independently, withdifferent depth and materials, the fabrication process of thesemiconductor chip is simplified as a whole.

[0115] In either method shown in FIGS. 7 or 8, epitaxial growth andformation of trench capacitors, which accompany thermal processing at ahigh temperature, are carried out after the isolations are formed in theSOI device region. Therefore, stress in the SOI device region can bereduced.

[0116] Devices can be fabricated at a uniform level over the bulk deviceregion 11 and the SOI device region, as in the previous embodiments.

[0117] Since boundary isolation 75 a, which belongs to either the bulkdevice region 11 or the SOI device region 12, is provided in advancebefore epitaxial growth, the side face of the SOI layer 73 isautomatically protected without requiring an additional step for forminga sidewall protection film.

[0118] Removing both the boundary isolation 75 a and the buried oxide 72by wet etching can avoid damage to the silicon base substrate 71 whenrevealing the top surface thereof.

[0119] <Fourth Embodiment>

[0120]FIGS. 9A and 9B illustrate semiconductor chips according to thefourth embodiment of the invention. The fourth embodiment is acombination of the second and third embodiments. Isolation deeper thanthe buried oxide is provided at the boundary, which allows a sidewallprotection film to be completely removed together with the surroundingarea containing crystal defect, and at the same time, isolations areoptimized in both the bulk device region and the SOI device region.

[0121] In the example shown in FIG. 9A, a semiconductor chip 90Acomprises a base substrate 91, a bulk device region 11 having anepitaxial growth layer 96 positioned on the base substrate 91, and anSOI device region 12 having a buried oxide 92 on the base substrate 91and an SOI layer 93 on the buried oxide 92. Devices 94, 98 arepositioned in the epitaxial growth layer 96 in the bulk device region11, and devices (SOIMOSFET) 45 are positioned in the SOI device region12. The semiconductor chip 90A also has first isolations 95 a separatingthe devices 94, 98 in the bulk device region 11, second isolations 95 bseparating the device 45 in the SOI device region 12, and a thirdisolation positioned at the boundary between the bulk device region 11and the SOI device region 12. The second isolation 95 b positioned inthe SOI device region 12 is shallower than the first and thirdisolations 95 a and 95 c. The third isolation 95 c positioned at theboundary is deeper than the buried oxide 92, and can prevent crystaldefect, which may occur at the edge of the buried oxide 92 or thesidewall protection film, from spreading toward the bulk device region11.

[0122] The device-fabrication surfaces of the bulk device region 11 andthe SOI device region 12 align with each other, and therefore, a DRAMcell 98 and other circuit elements 94 arranged in the bulk device region11 and an SOIMOSFET 45 positioned in the SOI device region 12 are at asubstantially uniform level.

[0123] To fabricate the semiconductor chip 90A, those steps up tofabricating DRAM trench capacitors are the same as those shown in FIGS.3A through 3E, or in FIGS. 7A through 7F. Then, first isolations 95 a inthe bulk device region 11 and a third isolation 95 c located at theboundary are formed using the same lithography process. If the stepsshown in FIGS. 3A through 3E arc employed, the sidewall protection filmand its surroundings containing damaged silicon are removed at once whenforming the third isolation 95 c. Second isolations 95 b are formed inthe SOI device region 12 by a separate lithography process even if theprocesses shown in FIGS. 3A through 3E are used. In the example shown inFIG. 9A, the third isolation 95 c slightly overlaps the buried oxide 92,getting into the SOI layer 93. However, it may slightly project into theburied oxide 92, depending on the etching condition. If the endmostsecond isolation 95 b in the SOI device region 12 is located very closeto the boundary, the endmost second isolation 95 b and the boundaryisolation (i.e., the third isolation) 95 c may overlap each other,resulting in the third isolation 95 with the same structure as thatshown in FIG. 9A.

[0124] In the example in FIG. 9B, a semiconductor chip 90B has a firstisolation 97 a separating a DRAM cell 98 in the bulk device region 11, asecond isolation 97 b separating an SOIMOSFET 45 in the SOI deviceregion 12, and a third isolation 97 c located at the boundary. The firstand second isolations 97 a and 97 b are of substantially the same depth,and are shallower than the third isolation 97 c. The third isolation 97c is set to be deeper than the buried oxide 92 in order to remove thesidewall protection film (not shown) together with the nearby damagedsilicon.

[0125] The first isolation 97 a for the DRAM cell 98 is as shallow asthe second isolation 97 b separating the SOI device 45 in order toreduce the plug resistance of the storage node electrode 29 of thetrench capacitor 30. In this manner, isolations in both the bulk deviceregion 11 and the SOI device region 12 are optimized. The fourthisolations 97 d for separating the peripheral MOSFETs 94 may befabricated at the same depth and at the same time as the third isolation97 c using the same material. Alternatively, the fourth isolation 97 dmay be formed together with the fist and the second isolations 97 a and97 b.

[0126] In either example of FIGS. 9A or 9B, the devices (e.g. DRAM cells98) arranged in the bulk device region 11 and the devices (e.g,SOIMOSFETs 45) arranged in the SOI device region 12 are positioned at auniform level.

[0127] Because the sidewall protection film and the nearby area in thebulk growth layer 96, which may have been damaged, are removed at oncewhen forming isolations, problems caused by stress can be eliminated. Inaddition, isolations can be optimized in both the bulk device region andthe SOI device region, and operation reliability is improved. Theadvantages of avoiding an increase in chip size and of the uniform levelof the device-fabrication surfaces are the same as those in the thirdembodiment.

[0128] <Fifth Embodiment>

[0129]FIG. 10 illustrates a semiconductor chip 100 according to thefifth embodiment of the invention.

[0130] Semiconductor chip 100, which is a system-on-chip having multiplefunction blocks, makes more efficient use of the bulk device region 11when DRAM cell with trench capacitors are formed in the bulk deviceregion 11. Consequently, the storage capacitance of the DRAM isincreased without increasing the area size, or in other words, thetrench capacitors are arranged at a higher density.

[0131] The semiconductor chip 100 comprises a base substrate 101, a bulkdevice region 11 having an epitaxial growth layer 106 formed on the basesubstrate 101, and an SOI device region 12 having a buried oxide 102positioned on the base substrate 101 and an SOI layer positioned on theburied oxide 102. In the bulk device region 11, DRAM cells 143 areformed. In the SOI device region 12, SOIMOSFET devices 45 are formed inthe SOI layer 103. The semiconductor chip 100 also has a first isolation105 a separating the DRAM cell 143 in the bulk device region 11, asecond isolation 105 b separating the MOSFET 45 in the SOI device region12, and a third isolation 105 c located at the boundary between the bulkdevice region 11 and the SOI device region 12. In the bulk device region11, the peripheral MOSFET 144 is isolated by the fourth isolation 105,which is of the same depth as the third isolation 105 c.

[0132] In the example shown in FIG. 10, the third isolation 105 c ispositioned at the boundary independently, and is set to be deeper thanthe buried oxide 102. However, as illustrated in the third embodimentreferring to FIG. 6, either the endmost second isolation 105 b in theSOI device region 12 or the endmost fourth isolation 105 d in the bulkdevice region 11 may function as the boundary isolation.

[0133] The DRAM cell 143 formed in the bulk device region 11 has atrench capacitor 130 consisting of a lower part (or first part) locatedunder the interface between epitaxial growth layer 106 and the basesubstrate 101, and an upper part (or second part) located above theinterface (i.e., in the epitaxial growth layer 106). The width or thelateral cross-sectional area of the lower part is greater than that ofthe upper part. The lower part (or the first part) of the trenchcapacitor 130, which is positioned under the interface with the bulkgrowth layer 106, extends into at least a part of the region directlybelow the gate 39 a of the DRAM cell transistor.

[0134] By expanding the trench capacitor 130 toward the region directlybelow the cell transistor, the storage capacitance can be increasedwithout increasing the area size of the DRAM sell array (not shown).

[0135] The semiconductor chip 100 having the above-described capacitorstructure makes good use of the SOI/bulk substrate fabrication processrequired for a system-on-chip. In order to fabricate a SOI/bulksubstrate, a predetermined area of the SOI layer 103 and the buriedoxide 102 are removed from the SOI substrate to reveal the basesubstrate 101. Then, the epitaxial growth layer 106 is grown from theexposed surface of the base substrate 101. If the capacitor structureshown in FIG. 10 is realized in a DRAM chip using a bulk substrate, thelower part (or first part) of the trench capacitor is formed in the bulksubstrate, and then, a silicon layer has to be epitaxially grown on thebulk substrate by a separate process in order to form the upper part (orsecond part). However, in the system-on-chip 100, the profile of thetrench capacitor can be adjusted into a desirable form by making use ofthe fabrication process of the SOI/bulk substrate, and therebyincreasing the storage capacitance.

[0136]FIGS. 11A through 11G illustrate a fabrication process of thesemiconductor chip (system-on-chip) 100.

[0137] (a) First, a shown in FIG. 11A, a mask pattern 104 made of anarbitrary mask material (e.g., SiO₂, SiN, Si₃N₄) or a composite materialis formed on the SOI wafer consisting of a silicon (Si) base substrate101, a buried oxide 102, and an SOI layer 103. In the area that is notcovered with the mask pattern 104, the SOI layer 103 and the buriedoxide 102 are removed by, for example, RIE in order to partially revealthe base substrate 101. The last stage for removing the buried oxide 102may be conducted by wet etching in order to minimize damage to thesilicon base substrate 101.

[0138] (b) Then, as shown in FIG. 11B, a lower part (or first part) ofthe DRAM trench capacitor is formed in the exposed silicon basesubstrate 101. For example, a trench is formed by RIE, and a lowerdiffusion plate 131 is formed. Dielectric (not shown) is formed in thetrench, and the trench is filled with polysilicon up to the middle.Collar oxide 132 is formed, and the trench is further filled with thepolysilicon to form the storage node electrode 129, which is to be thelower part (or first part) of the trench capacitor extending into thebase substrate 101.

[0139] (c) Then, as shown in FIG. 11C, a stopper 133 and a sidewallprotection film 134 are formed to cover the lower part of the trenchcapacitor and the exposed side face of the SOI layer 103, respectively.The stopper 133 and the sidewall protection film 134 are made of siliconnitride or other suitable material.

[0140] (d) Then, as shown in FIG. 11D, an epitaxial growth layer 106 isformed on the exposed base substrate 101 so as to be in alignment withthe top face of the SOI layer 103. The epitaxial growth layer 106 isformed by selective epitaxial growth using, for example,dichroloresilane (SiH₂Cl₂) as a source gas.

[0141] (e) Then, as shown in FIG. 11E, a mask pattern 114 is formed overthe bulk device region 11 and the SOI device region 12, and an openingthat reaches the first part of the trench capacitor 130 is formed in theexpitaxial growth layer 106. The width or the lateral cross-sectionalarea of the opening is smaller than that of the first part formed in thebase substrate 101. A collar dioxide 135 is formed on the sidewall ofthe opening, and the opening is filled with polysilicon to form a secondpart (or an upper part) of the trench capacitor 130. The second partelectrically connects the first part to the cell transistor that is tobe formed above the trench capacitor 130.

[0142] (f) Then, as shown in FIG. 11f, fist isolations 105 a positionedin the bulk device region 11, second isolations 105 b positioned in theSOI device region 12, and a third isolation 105 c located at theboundary between the bulk device region 11 and the SOI device region 12are formed. When forming the third isolation 105 c, the sidewallprotection film 134 and the surrounding area containing damaged siliconare removed. Although no shown in FIG. 11F, isolations for theperipheral circuit (not shown), which may be formed in the bulk deviceregion 11, are also formed at this stage. The third isolation 105 c maybe formed by a separate process from the first and second isolations 105a, 105 b. Because the width of the second part (i.e., the upper part) ofthe trench capacitor 130 formed in the bulk growth layer 106 isrelatively small, it is desirable for the first isolation 105 a to be asshallow as the second isolation 105 b in the SOI device region 12.Alternatively, they may be formed together with the first and secondisolations 105 a and 105 b in the same process for simplicity of theprocess.

[0143] (g) Then, as shown in FIG. 11G, DRAM cell transistors 143 andSOIMOSFETs 45 are formed in the bulk device region 11 and the SOI deviceregion 12, respectively. Transistors constituting the peripheral circuit144 may also be formed, thereby producing the semiconductor chip 100illustrated in FIG. 10.

[0144]FIG. 12 illustrates a modification of the semiconductor chip 100.The modification shown in FIG. 12 is a combination of the DRAM cellstructure of the fifth embodiment shown in FIG. 10 and the boundarystructure of the third embodiment shown in FIG. 6. The modification ofFIG. 12 also includes a structural example of multilevel interconnectionformed on or above the device-fabrication surfaces of the bulk deviceregion 11 and the SOI device region 12.

[0145] The semiconductor chip shown in FIG. 12 has first isolations 107a separating the DRAM cells 143 in the bulk device region 11, and secondisolations 107 b, 107 c separating the devices (e.g., SOIMOSFETs) 45 inthe SOI device region 12. The endmost isolation 107 c in the SOI deviceregion 12 functions as a boundary layer.

[0146] Bit lines 125 and word lines are provided above the DRAM cells143. The drain of the DRAM cell 143 is connected to the associated bitline 125 via the bit-line contact plug 124. In the SOI device region 12,silicide 123 is provided on the source and drain of the SOIMOSFET 45 forthe purpose of reducing resistance. The SOIMOSFFT 45 is connected to theupper layer Al interconnection 127 via the plug 128.

[0147] To fabricate the semiconductor chip shown in FIG. 12, the secondisolations 107 b, 107 c are formed in the SOI device region 12 inadvance, and the silicon base substrate 101 is revealed in thepredetermined area using either the process illustrated in FIGS. 7A and7B or the process shown in FIGS. 8A and 8B. Then, a relatively widefirst part of the trench capacitor is formed in the exposed silicon basesubstrate 101. Then, the stopper 133 is placed on the top face of thefirst part of the trench capacitor 130, and bulk growth layer 106 isformed by, for example, selective epitaxial growth on the silicon basesubstrate 101. During the epitaxial growth, the side face of the SOIlayer 103 is protected by the endmost second isolation 107 c positionedat the boundary, which automatically functions as a sidewall protectionfilm.

[0148] Then, the second part of the trench capacitor 130, which isnarrower than the first part, is formed in the epitaxial growth layer106. The first isolations 107 a are also formed in the epitaxial growthlayer (i.e., in the bulk device region 11). Then, DRAM cells 143 (FIG.10), SOIMOSFETs 45, and devices 144 of the peripheral circuit are formedin the respective regions 11 and 12. The protection film 122 is formedover the entire surface, and the silicide 123 is provided on the sourceand drain of the SOIMOSFET 45 where the protection film is removedselectively. The interlevel dielectric 121 is deposited. The bit-linecontact plug 124 for connecting the drain of the DRAM cell transistor143 to the bit line 125, and the plug 128 for connecting the source andthe drain of the SOIMOSFET 45 to the upper-level interconnection, areformed. Finally, upper-level interconnections 125, 126 and 127 areformed using an ordinary technique.

[0149] In the example shown in FIG. 12, the first part of the trenchcapacitor 130 expands toward the region directly below the DRAM celltransistor, making good use of the silicon base substrate 10. Thisarrangement allows the storage capacitance or the arrangement density ofthe trench capacitor to be increased without increasing the DRAM arraysize. In addition, the endmost second isolation 107 c functions as theboundary layer. Consequently, stress at the boundary is reduced, andincrease in chip size is prevented. Selective epitaxial growth allowsthe device fabrication surfaces of the bulk growth layer 106 and the SOIlayer 103 to be in alignment with each other, and the devices belongingto different functional blocks can be arranged at a uniform level.

[0150] <Sixth Embodiment>

[0151]FIG. 13 schematically illustrates the semiconductor chip 200according to the sixth embodiment of the invention. The semiconductorchip 200 comprises a silicon base substrate 201, a bulk device region 11having a bulk growth layer (e.g., an epitaxial growth layer) 206 on thebase substrate 201, an SOI device region 12 having a buried oxide 202 onthe base substrate 201 and an SOI layer 203 on the buried oxide 202, aboundary layer 407 located at the boundary between the bulk deviceregion 11 and the SOI device region 12, and a dummy capacitor (or adummy pattern) 240. The dummy capacitor 240 is positioned in the bulkdevice region 11 near the boundary.

[0152] The semiconductor chip 200 also has DRAM cells 213 and peripheralMOSFETs 214 positioned in the bulk device region 11, and SOIMOSFETs 216positioned in the SOI device region 12. The device-fabrication surfaceof the bulk growth layer 206, in which the DRAM cells 213 and theperipheral MOSFETs 214 are formed, and the device-fabrication surface ofthe SOI layer 203, in which the SOIMOSFET 216 is formed, are atsubstantially the same level.

[0153] The devices in the bulk device region 11 are separated from oneanother by first isolations 205 a, and the devices in the SOI deviceregion are separated from one another by second isolations 205 b. Thedepth of the first and second isolations 205 a and 205 b aresubstantially the same in the example shown in FIG. 13. However, thesecond isolation 205 b may be set to be shallower as long as it reachesthe buried oxide 202. By selecting such an etching condition that theetching rate with respect to silicon is much smaller than that fordioxide, the first and second isolations 205 a and 205 b with differentdepths are formed by the same process.

[0154] The depth of the dummy capacitor 240 is set to be deeper than theburied oxide 202 of the SOI device region 12. Even if dislocation occursat the boundary between the bulk device region 11 and the SOI deviceregion 12 and advances toward the bulk device region 11 as indicated bythe arrow A, the dummy capacitor 240 stops the dislocation formexpanding into the bulk device region 11.

[0155] In the example shown in FIG. 13, the dummy pattern is formed as adummy capacitor 240 having the same shape and structure as the trenchcapacitor 240 of the DRAM cell 213 positioned in the bulk device region11. Accordingly, the dummy capacitor 240 is filled with the samematerial as the storage electrode 229, and has diffusion layer (or lowerelectrode) 231 and collar dioxide 217. However, the dummy 240 does notnecessarily have diffusion layer 231 or collar dioxide 217.Alternatively, the dummy capacitor 240 may be furnished with isolation,for example, the first isolation 205 a, in order to make the dummyelectrically inactive.

[0156] Such a dummy structure is applicable to all of the previousembodiments. For example, although the semiconductor chip 200 shown inFIG. 13 has a boundary layer 207, as in the embodiment 1 shown in FIG.2, an independent isolation 65 c may be positioned at the boundary asthe boundary layer. In this case, the dummy capacitor 240 is located inthe bulk device region 11 near the boundary isolation 65 c. In addition,the endmost isolation 75 a in the SOI device region 12 may function asthe boundary layer, as illustrated in FIG. 7. In this case, the dummycapacitor is again located in the bulk device region near the endmostisolation 75 a. If the trench capacitor has the two-part structure shownin FIG. 10, the dummy capacitor 240 may be formed in the same shape andstructure as the trench capacitor 130. However, the dummy may, ofcourse, be formed in a different shape from the trench capacitor 130(for example, without the upper electrode). In any cases, the dummycapacitor 240 is deeper than the buried oxide 202 of the SOI deviceregion 12.

[0157] The dummy pattern is formed immediately after the epitaxialgrowth layer 206 is formed. If the system-on-chip has a trench-capacitorDRAM macro as a functional block in the bulk device region, asillustrated in the embodiments, it is desirable to form the dummycapacitor by the same process used in fabricating the trench capacitors.

[0158]FIG. 14 illustrates a layout example of the dummy capacitor 240shown in FIG. 13. In this example, dummy capacitors 240, which have thesame structure as the DRAM cell trench capacitors 230, are arrangedalong the boundary between the bulk device region 11 and the SOI deviceregion 12. As has been mentioned above, the dummy capacitor 240 may notnecessarily have the same structure as the trench capacitors 230.However, the similarity in structure allows the process conditions to beagreed with those for the fabrication of the memory cells. The depth ofthe dummy capacitor 240 is set to be deeper than the buried oxide in theSOI device region 12.

[0159]FIG. 15 illustrates modifications of the dummy pattern forpreventing expansion of dislocation more effectively. FIG. 15A showsline dummy 310 surrounding the macro (e.g., DRAM macro) in the bulkdevice region 11, and FIG. 15B shows island dummies 311 surrounding themacro. In either example, the dummy pattern is formed immediately afterthe bulk growth layer (e.g., the epitaxial growth layer) is formed inthe bulk device region 11. If the bulk device region 11 includes a DRAMmacro as a functional block, the dummy patterns 310 and 311 are formedwhen the DRAM trench capacitors are formed. In this case, the line widthof the line dummy 310 is desirably set to be in agreement with the shortside of the capacitor pattern of the DRAM cell. Similarly, the shortside of the island dummy 311 is desirably set to be in agreement withthe short side of the capacitor pattern. This arrangement allows thedummy pattern to be formed by the same process used in fabricating thememory cells with large process margin.

[0160] The semiconductor chip 200 having the dummy patterns near theboundary can effectively prevent dislocation from expanding into thebulk device region from the boundary, in addition to those advantages ofreduction of stress, level uniformity of the device-fabricationsurfaces, and prevention of increase in chip size.

[0161] <Other Embodiments>

[0162] In the first through fifth embodiments, the bulk growth layer isformed by selective epitaxial growth of single crystal silicon. However,the bulk growth layer may be formed of silicon germanium (SiGe.) usingepitaxial growth. Furthermore, the bulk growth layer may be formed bynon-selective epitaxial growth.

[0163] The semiconductor chip may include two or more different types ofbulk device regions. For example, a silicon bulk growth layer and asilicon germanium bulk growth layer may be arranged in an SOI substrate.In this case, it is desirable that the boundary between the SOI deviceregion and each of the bulk device region, or between the bulk deviceregions, is filled with a gate electrode material, such as polysilicon,SiGe, or other silicon-based coumpound semiconductor, used for thedevices fabricated in the associated bulk device region. Thisarrangement can reduce stress and increase the design margin.

[0164] The endmost isolation positioned closest to the boundary betweenthe SOI device region and the silicon or SiGe bulk device region mayfunction as the boundary layer. This arrangement can reduce dead spacein the chip.

[0165] For example, the semiconductor chip may have a DRAM macro in thesilicon bulk region, a bipolar circuit in the SiGe bulk device region,and a logic circuit in the SOI device, which are all mounted on a singlechip. The isolations in the respective bulk device regions and the SOIdevice region can be optimized, depending on the characteristics of thedevices or the functional block formed in these regions, as illustratedin the third through fifth embodiments. Optimization of the isolationsrealizes a high-performance system LSI.

[0166] Dummy patterns deeper than the buried oxide of the SOI substratemay be arranged in both the silicon bulk device region and the SiGe bulkdevice region along the boundary in order to prevent dislocation, whichis likely to occur at the boundary, from expanding into the bulk deviceregions. The dummy pattern may be a dummy trench having the samestructure as a trench capacitor when the bulk device region includestrench capacitors. Alternatively, if the bulk device region includesvertical bipolar transistors, the dummy pattern may be a deep trenchhaving the same structure as the deep isolation for separating thecollector of the bipolar transistor. The buried insulator of the SOIsubstrate is not limited to a buried oxide.

[0167] All the embodiments imply various modifications and substitutionspossible by adjusting the etching conditions of trench isolations. Forexample, in the second embodiment shown in FIG. 4, the isolations 65 a,65 b, and 65 c are formed in both the bulk device region and the SOIdevice region at the same time by setting an etching condition wheresilicon and dioxide are etched at the same etching rate. However, if anetching condition where the etching rate with respect to dioxide isslower than that for silicon is selected, the second isolation 65 b inthe SOI device region 12 becomes shallower than the first isolation 65 ain the bulk device region 13. To be more precise, the isolation 65 clocated at the boundary becomes asymmetric. In other words, a part ofthe isolation 65 c positioned on the buried oxide 52 is of the samedepth as the second isolation 65 b in the SOI device region 13, and theother part of the isolation 65 c is as deep as the first isolation 65 ain the bulk device region 11. It is preferable for the isolations 65 aand 65 c to be deeper than the interface between the base substrate 51and the buried oxide 52 for the purpose of completely removing thedamaged portion of the bulk growth layer near the boundary due to theadverse influence of the sidewall protection film or crystal defect.

[0168] The isolations 65 a, 65 c positioned in the bulk device region 11may be fabricated in a separate process from the fabrication process forthe isolation 65 b in the SOI device region 12. For example, theisolations 65 a and 65 c are fabricated together under the etchingcondition where silicon and dioxide are etched at the same etching rate,and the isolation 65 b is formed under a different condition where theetching rate with respect to dioxide is slower than that for silicon. Insuch a case, the boundary isolation 65 c becomes asymmetric, and theisolation 65 b in the SOI device region 12 is optimized. The shallowtrench of the isolation 65 b is easily and precisely filled with aninsulator, allowing miniaturized isolation patterns of the logic circuit

[0169] Furthermore, the uniformity of the device-fabrication surfacesover the bulk device region and the SOI device region can preventadverse effect on subsequent processes. An appropriate boundary layerreduces undesirable stress and the resultant crystal defect at theboundary between the bulk device region and the SOI device region. Theimproved layout arrangement of the isolations located on and near theboundary can prevent an increase in chip size.

What is claimed is:
 1. A semiconductor chip comprising: a basesubstrate; a bulk device region having a bulk growth layer on a part ofthe base substrate, the bulk device region having a firstdevice-fabrication surface in which a bulk device is positioned on thebulk growth layer; an SOI device region having a buried insulator on theother part of the base substrate and an SOI layer on the buriedinsulator, the SOI device region having a second device-fabricationsurface in which an SOI device is positioned on the SOI layer, the firstand second device-fabrication surface being positioned at asubstantially uniform level; and a boundary layer located at theboundary between the bulk device region and the SOI device region. 2.The semiconductor chip according to claim 1, wherein the bulk growthlayer is a silicon bulk growth layer, and the boundary layer reaches thebase substrate and is made of one of polysilicon or silicon-basedcompound semiconductors.
 3. The semiconductor chip according to claim 1,wherein the bulk device region includes a first isolation separating thebulk device, and the SOI device region includes a second isolationseparating the SOI device, the first and second isolations being ofsubstantially the same depth.
 4. The semiconductor chip according toclaim 3, wherein the first and second isolations have a depth reachingthe buried insulator.
 5. The semiconductor chip according to claim 4,wherein the bulk device region has a pn junction positioned above theinterface between the base substrate and the bulk growth layer.
 6. Thesemiconductor chip according to claim 1, further comprising a firstisolation in the bulk device region, a second isolation in the SOIdevice region, and a third isolation positioned at the boundary andfunctioning as the boundary layer, the first, second, and thirdisolations being of substantially the same depth.
 7. The semiconductorchip according to claim 6, wherein the first, second, and thirdinsulators are deeper than the buried insulator.
 8. The semiconductorchip according to claim 7, wherein the third insulator has a sidewallthat is in contact with the buried insulator.
 9. The semiconductor chipaccording to claim 7, wherein the bulk device region has a pn junctionpositioned below the interface between the base substrate and the bulkgrowth layer.
 10. The semiconductor chip according to claim 1, furthercomprising a first isolation in the bulk device region, a secondisolation in the SOI device region, and a third isolation positioned atthe boundary and functioning as the boundary layer, the second isolationbeing shallower than the third isolation.
 11. The semiconductor chipaccording to claim 1, further comprising a first isolation in the bulkdevice region, and a second isolation that is shallower than the firstisolation, wherein the boundary layer is whichever the first or thesecond isolation that is positioned closest to the boundary.
 12. Thesemiconductor chip according to claim 11, wherein the second isolationfunctions as the boundary layer, and has a bottom face that is incontact with the buried oxide.
 13. The semiconductor chip according toclaim 1, further comprising a dummy pattern in the bulk device regionnear the boundary.
 14. The semiconductor chip according to claim 13,wherein the bulk device positioned in the bulk device region includes aDRAM cell having a trench capacitor, and the dummy pattern is a dummycapacitor.
 15. The semiconductor chip according to claim 1, wherein thebulk device positioned in the bulk device region includes a DRAM cellhaving a trench capacitor, the trench capacitor comprising a first partextending at and below the interface between the base substrate and thebulk growth layer, and a second part extending above the interface, thewidth of the first part being greater than that of the second part. 16.A method for fabricating a semiconductor chip comprising: preparing anSOI substrate consisting of a base substrate, a buried insulator on thebase substrate, and a silicon layer on the insulator; removing thesilicon layer and a portion of the buried insulator in a predeterminedarea of the SOI substrate; forming a sidewall protection film covering aside face of the silicon layer revealed by the removal; exposing thebase substrate in said predetermined area; forming a bulk growth layerfrom the exposed surface of the base substrate until the top face of thebulk growth layer is in alignment with the top face of the siliconlayer; forming isolations having the same depth in the bulk growth layerand in the SOI substrate in a same process; and fabricating devices inthe bulk growth layer and the silicon layer on the buried insulator. 17.The method according to claim 16, wherein the base substrate is exposedby wet etching.
 18. The method according to claim 16, furthercomprising: removing the sidewall protection film after the formation ofthe bulk growth layer, wherein the fabrication of devices includesforming a gate and filling a recess resulting from the removal of thesidewall protection film with a semiconductor gate material.
 19. Themethod according to claim 18, wherein the semiconductor gate materialincludes polycilicon and silicon-based compound semiconductors.
 20. Themethod according to claim 16, wherein the formation of isolationsincludes providing isolation at the boundary between the bulk growthlayer and the SOI substrate, while removing the sidewall protectionfilm.
 21. A method for fabricating a semiconductor chip comprising:preparing an SOI substrate consisting of a base substrate, a buriedinsulator on the base substrate, and a silicon layer on the insulator;removing the silicon layer at a first position of the SOI substrate toform a first isolation in the removed portion, a side face of thesilicon layer being covered with the first isolation; exposing a topface of the base substrate at a second position, while keeping the sideface of the silicon layer covered with the first isolation; forming abulk growth layer from the exposed surface of the base substrate untilthe top fate of the bulk growth layer is in alignment with the top faceof the silicon layer; forming a second isolation in the bulk growthlayer, the second isolation being deeper than the first isolation; andfabricating devices in the bulk growth layer and the silicon layer onthe buried insulator.
 22. The method according to claim 21, wherein theformation of the first isolation includes providing an isolation at aposition that is to be a boundary between the bulk growth layer and theSOI substrate.
 23. The method according to claim 21, wherein theformation of the first isolation includes providing an isolation to thesec.0ond position at which the base substrate is to be exposed.
 24. Themethod according to claim 21, further comprising: forming a first partof a trench capacitor in the exposed surface of the base substratebefore the bulk growth layer is formed, the first part having a firstwidth; and forming a second part of the trench capacitor in the bulkgrowth layer, the second part being connected to the first part andhaving a second width narrower than that of the first part.
 25. A methodfor fabricating a semiconductor chip comprising: preparing an SOIsubstrate consisting of a base substrate, a buried insulator on the basesubstrate, and a silicon layer on the insulator; removing the siliconlayer and the buried oxide in a predetermined area of the SOI substrateto expose the base substrate; forming a first part of a trench capacitorin the exposed base substrate; forming a bulk growth layer on theexposed base substrate until the top face of the bulk growth layer is inalignment with the top face of the silicon layer on the buriedinsulator, and forming a second part of the trench capacitor connectedto the first part and having a second width narrower than the firstwidth.
 26. The method according to claim 25, further comprising: formingisolations in the bulk growth layer and in the SOI substrate in the sameprocess.
 27. A method for fabricating a semiconductor chip comprising:preparing an SOI substrate consisting of a base substrate, a buriedinsulator on the base substrate, and a silicon layer on the insulator;removing the silicon layer and the buried oxide in a predetermined areaof the SOI substrate to expose the base substrate; forming a bulk growthlayer on the exposed base substrate until the top face of the bulkgrowth layer is in alignment with the top face of the silicon layer onthe buried insulator; forming a dummy pattern in the bulk growth layernear the boundary with the SOI substrate, the dummy pattern being deeperthan the buried insulator; and fabricating devices in the bulk growthlayer and in the SOI substrate.
 28. The method according to claim 27,wherein the formation of the dummy pattern includes forming a trenchcapacitor in the bulk growth layer in the same process as for formingthe dummy pattern.